Field of the Invention
This invention relates to a circuit for discriminating an analog picture signal by binary-encoding.
Description of the Prior Art
Usually an analog picture signal provided by an image pickup device or by analog transmission from, e.g., a facsimile device is binary-encoded by a picture signal discriminating circuit before it is stored or subjected to logical operation. When the analog picture signal passes through the image pickup device's optical system, however, its modulation transfer function (MTF) deteriorates mainly due to aberration of the system's optical lens or the arrangement of photoelectric conversion elements in the image pickup device. When the analog picture signal with this deteriorated MTF is then binary-encoded by the picture signal discriminating circuit, it becomes impossible to discriminate the density accurately in correspondence to the original picture.
FIG. 1(a) shows the picture data of a "line" of the original. Black regions 11 on a white background are shaded with oblique lines. FIG. 1(b) shows a typical analog picture signal 12 which is outputted by an image pickup device (such as an image sensor) reading the lens in FIG. 1(a). With analog picture signal 12's deteriorated representation of the original picture, there can be no high fidelity regardless of how the binary-encoding level is set. For instance, when a relatively high binary-encoding level S.sub.1 is selected, a binary-encoded signal 14 as shown in FIG. 1(c) is obtained. It is difficult with such a high level to read white picture data P.sub.1 satisfactorily on the black background. On the other hand, when a relatively low binary-encoding level S.sub.2 is selected, a binary-encoded signal 15 as shown in FIG. 1(d) is obtained making it difficult to read black picture data P.sub.2 satisfactorily on a white background. This deterioration also occurs with a ladder pattern in which white and black thin lines appear alternately like a ladder.
The analog picture signal provided by the image pickup device has been described. Whereas, in a facsimile device or the like, an analog picture signal provided by analog transmission is binary-encoded for recording, the same deterioration occurs because of picture quality deterioration in the transmission path.
In view of the foregoing, an object of this invention is to provide a picture signal discriminating circuit in which binary-encoding discrimination is achieved with higher fidelity.